The frequency spectrum that is shared among radio communication devices is limited. Thus the ability of a transmitter to transmit as much information as possible in an allocated frequency spectrum or channel without interfering with other communication devices in adjacent channels is of great importance. To transmit as much information as possible in the allocated channel, digital communication systems typically modulate both the amplitude and phase of a radio frequency (RF) carrier. The amplitude modulation allows more information to be encoded on the carrier in a given channel than if only the phase was modulated. However, the amplitude modulation puts additional requirements on the transmitter that would not exist if only the phase of the RF carrier was modulated.
These additional requirements are due to the inherent nonlinear effects resulting from the amplification of an amplitude modulated signal by an RF power amplifier. Due to the nonlinear characteristics of the RF power amplifier, an additional magnitude component and phase component are added to the original signal. These additional components are due to the amplitude compression characteristics (AM/AM) and the phase distortion (AM/PM) characteristics of the RF power amplifier when it is driven over a range of amplitudes. If these distortion characteristics are not compensated they will cause spreading of the spectrum into the adjacent channels and thus interfere with communication devices using adjacent channels.
A number of prior art signal processing techniques have been developed to compensate for the nonlinear characteristics of power amplifiers. Among these techniques are predistortion, adaptive predistortion, feedforward correction loops and feedback correction loops. Predistortion and adaptive predistortion seek to inject a predistorted signal into an input signal's path prior to amplification. The predistorted signal includes components equal and opposite to the distortion introduced by the power amplifier and is designed to cancel the distortion introduced to the input signal by the power amplifier. However, the application of a predistorted injection signal is limited due to the difficulty of characterizing a power amplifier and correcting for the power amplifier's characteristics with a predetermined function. Feedback and feedforward are real time correction techniques and therefore do not require characterization of the power amplifier. However, feedback correction has limited bandwidth performance due to loop stability requirements and the group delay of power amplifiers when the forward path includes multiple power amplifiers. Feedforward correction provides better bandwidth performance, but at the expense of costly error amplifiers, which may also introduce distortion into the feedforward path, and additional system expense and complexity such as carefully matched delays between the input signal forward path and the feedforward loop.
To overcome the deficiencies of the individual correction techniques, a hybrid correction technique was proposed by Belcher et al., in U.S. Pat. No. 5,760,646. Belcher uses an adaptive predistortion loop that determines and injects a predistorted signal into the input signal, and a feedforward correction loop to correct the residual non-linearities in the amplified output signal. However, the adaptive predistortion loop proposed by Belcher is a hardware implementation that generates a predistortion signal and then modulates the input signal, based on the predistortion signal, by use of a vector modulator. Belcher's adaptive predistortion loop is a feedback loop that continually feeds back the output signal and continually modulates the input signal based on the fed back output signal. This technique is expensive to implement, particularly as more coefficients are generated, requiring additional hardware, in an effort to improve the accuracy of the inverse function for the injected signal. Furthermore, the modulation of the input signal by a vector modulator introduces additional distortion and injects a variable group delay in the forward path of the input signal, which is undesireable in a system that also uses feedforward error correction. In addition, the technique proposed by Belcher performs all correction in the predistortion loop at RF. The predistortion injection technique ideally corrects distortion introduced by the RF power amplifier to the informational content of the signal that is to be transmitted. The informational content of the signal is the baseband signal that has been modulated onto an RF carrier. Precision may be lost when the baseband signal is first modulated onto an RF carrier and then the modulated RF carrier is corrected with predistortion.
Therefore a need exists for a method and apparatus for providing a predistortion signal, which method and apparatus generates a more accurate inverse function without incurring increased costs, does not introduce additional distortion or a variable delay into the input signal forward path, and is able to precisely define the desired characteristics of the signal being feed into the power amplifier.